Function: plotrpoint
Class: highlevel
Section: graphic
C-Name: rectrpoint
Prototype: vLGG
Help: plotrpoint(w,dx,dy): draw a point (and move cursor) at position dx,dy
 relative to present position of the cursor in rectwindow w.
Doc: draw the point $(x1+dx,y1+dy)$ on the rectwindow $w$ (if it is inside
 $w$), where $(x1,y1)$ is the current position of the cursor, and in any case
 move the virtual cursor to position $(x1+dx,y1+dy)$.
